1. Field of the Invention
The present invention relates the recovery and storage of hydrogen and, in particular, to hydride compositions and methods for storing hydrogen.
2. Discussion of Background
The hydrogen-absorbing materials known as hydrides are capable of absorbing large amounts of hydrogen which can then be desorbed under the appropriate temperature and pressure conditions. Hydrides are widely used in processes relying on the recovery, storage and supply of hydrogen, particularly in the hydrogen processing and energy conversion fields. Current uses include hydrogen storage, hydrogen pumping and compression, heat pumps, batteries, fuel cells and hydrogen isotopes purification and separation processes.
Known hydrides include pure metals (Mg, Ti, V, Nb, Pt, Pd, and so forth), alloys (the La-, Ti-, and Co- alloys, rare earth-Ni alloys), and various hydride-containing compositions. The capacity of a particular hydride to absorb or release hydrogen depends on the temperature and the external hydrogen gas pressure. The capacity of hydrides other than pure metals also depends on the surface area of the material. To maximize surface area and absorption/desorption efficiency, the material is often supplied in the form of fine-grained particles or pellets. The hydrogen-storage capacity of these materials is ultimately limited by the available surface area for hydrogen absorption.
Porous glass materials made by sol-gel processes have very large specific surface areas due to their high porosity. Typically, a solution (the sol solution) containing an organic liquid such as alcohol together with a metal oxide, alkoxide, alcoholate, sulfide or the like, is polymerized to obtain a gel. The alcohol replaces the water in the pores of the gel, and the gel is dried to remove the liquid phase and obtain a porous glass product. Drying is carried out in such a way as to minimize shrinkage and fracturing of the gel.
Drying at room temperature (about 20xc2x0 C.) and atmospheric pressure results in xerogels, which have porosities up to approximately 80%. Drying at supercritical temperature and pressure conditions results in aerogels, which have porosities up to approximately 90% or higher. Drying under supercritical conditions prevents formation of a meniscus between the liquid and gaseous phases, so the liquid can be removed without subjecting the gel structure to compressive forces due to the surface tension of the liquid-gas interface.
Sol-gel processes can produce an inert, stable product with a very large specific surface area, up to 1000 m2/g or higher. Advantages of sol-gel processes include low energy requirements, production of a high purity product, and uniform dispersion of additives into the product. See, for example, the processes for producing silica aerogels described by Blount (U.S. Pat. No. 4.954,327) and Zarzycki, et al. (U.S. Pat. No. 4,432,956).
A number of porous glass compositions have been developed. For example, an aerogel substrate may be loaded with tritium and combined with a radioluminescent composition, whereby the tritium is the energy source for the radioluminescent material (Ashley, et al.. U.S. Pat. No. 5,078,919). Porous glass is used in automobile catalytic converters as a support for metal catalysts (Elmer, et al., U.S. Pat. No. 3,802,647).
Porous glass compositions may be doped with metals or metal compounds, including metal alkoxides (Motoki, et al., U.S. Pat. No. 4,680,048), metals and alkoxides (Puyanxc3xa9, et al., U.S. Pat. No. 4,495,297), and metal oxides (Wada, et al., U.S. Pat. No. 4,978,641). Van Lierop, et al. (U.S. Pat. No. 4,806,328) add metal oxides to porous glass to adjust the refractive index.
Even with their high porosityxe2x80x94and correspondingly high specific surface areaxe2x80x94such porous glass compositions absorb only small amounts of hydrogen by volume. These materials are therefore not suitable for use as hydrogen absorbers.
There is a need for a composition that can reversibly absorb large amounts of hydrogen. The composition should have a high porosity to allow permeation of hydrogen gas, thereby contacting the material with hydrogen to facilitate absorption. Preferably, it should maintain its hydrogen-absorbing capacity over a large number of absorption/desorption cycles.
According to its major aspects and broadly stated, the present invention is a hydride composite prepared by a sol-gel process. The starting material is an organometallic compound such as tetraethoxysilane. A sol is prepared by mixing the starting material, alcohol, water, and an acid. The sol is conditioned to the proper viscosity and a hydride in the form of a fine powder is added. The mixture is polymerized, then dried under supercritical conditions. The final product is a composition having a hydride uniformly dispersed throughout an inert, stable, highly porous matrix. The composition can be fabricated in the form of pellets or other shapes as needed for the particular application. The composition is capable of absorbing up to approximately 30 moles of hydrogen per kilogram at room temperature and pressure, rapidly and reversibly. Hydrogen absorbed by the composition can be readily be recovered by heat or evacuation. Uses for the composition include hydrogen storage and recovery, recovery of hydrogen from gas mixtures, and pumping and compressing hydrogen gas.
An important feature of the present invention is the matrix, made by a sol-gel process. A first mixture containing approximately two to five parts alcohol to one part of water is prepared. The acidity of the mixture is adjusted to the approximate range of 1.0 to 2.5 by adding an acid. A second mixture is prepared by mixing approximately one part alcohol to two parts of an organometallic compound such as an alkoxysilane, particularly tetraethoxysilane ((C2H5O)4Si). Alternatively, organometals of metals of the forms MOxRy and M(O)x, where R is an alkyl group of the form CnH2n+1, M is an oxide-forming metal, n, x, and y are integers, and y is two less than the valence of M, may be used. The first mixture is slowly added to the second, then the resulting solution is conditioned until it reaches the approximate viscosity of heavy oil. A hydride in the form of fine particles is added. Other additives such as foaming agents and stabilizers may also be added to the mixture. The mixture is polymerized to obtain a gel that contains the polymerized material and a liquid as two continuous phases. The gel is dried under supercritical conditions to remove the liquid phase. Drying under supercritical conditions can yield a composition with a porosity of 90% or higher. Drying may alternatively be carried out in air, or in other atmospheres including inert atmospheres. The optimum conditions and drying time are best determined by observation and a modest degree of experimentation for each particular composition.
An additional feature of the present invention is the hydride. The hydride is preferably a transition metal hydride such as Al, Cu, La, Ni. Pd, Pt, or combinations thereof, and most preferably Pt or a Laxe2x80x94Nixe2x80x94Al alloy. To maximize the surface area and catalyzing activity of the hydride, it is preferably supplied in the form of a powder having particles less than approximately 100 xcexcm in size, in an amount up to approximately 50 wt. % of the dry gel. After polymerization and drying, the composition includes the uniformly dispersed hydride in a porous matrix with a high specific surface area.
Another feature of the present invention is the combination of the matrix and the hydride. The small size of the hydride particles maximizes the available surface area of the hydride for both catalysis and hydrogen absorption. The high specific surface area of the matrix provides a large area for hydrogen absorption thereon. However, the surface of an aerogel normally absorbs only a small amount of hydrogen by itself. The combination of the aerogel matrix with the hydride produces an unexpected synergistic effect: the composition is capable of storing surprisingly large amounts of hydrogen, more than the mathematically combined capacity of the aerogel and the hydride. separately. Although not wishing to be bound by theory, it is believed that the hydride may act as a catalyst to improve the hydrogen-storage capability of the aerogel matrix. See U.S. patent application Ser. No. 07/967,653 filed Oct. 28, 1992 and hereby incorporated by reference for information related to this application.
Other features and advantages of the present invention will be apparent to those skilled in the art from a careful reading of the Detailed Description of a Preferred Embodiment presented below.